Detectives are questioning a man after a car with a baby inside was hijacked by a thief posing as a police officer, who returned half an hour later and dumped the child in the street.

The young parents of the four-month-old boy endured agonies during a "horrendous" wait after their silver Lexus careered off following a brief struggle.

The child's 24-year-old mother, who asked not to be named, managed to grab her two-year-old son from the back of the car but could not reach the baby. She and her partner ran for help after a last glimpse of the baby, strapped into his car seat, as the car took off. "I was frantic because of the speed he went off at," she said yesterday. "I knew he'd gone down a dead end, and people came out of offices and tried to block the street with their cars, but he squeezed through a gap and went off at about 70mph."

The couple were still in the street in Bolton when a Nissan Micra pulled up and a man jumped out with the baby still in the seat. He dumped the baby on the pavement without looking at the parents and drove off. The Micra crashed into two other cars shortly after the baby was returned. A 39-year-old man was arrested at the scene of the second accident, with leg and neck injuries. It emerged later that the Lexus had also been involved in a hit-and-run accident within minutes of the car-jacking. It was damaged but zig-zagged off and has not been recovered.

The arrested man was treated in hospital and is now being questioned on suspicion of kidnapping and theft. Two other people suffered minor injuries in the accident involving the Nissan Micra, which swerved into a Citroen Xsara and a Volkswagen Passat.